1. Field of the Invention
The present invention relates to a drive control apparatus and method for a motor such as a stepping motor, rotation of which is controlled by switching the excitation phases in turn, and an image forming apparatus using the method.
2. Description of the Related Art
An electrophotographic image forming apparatus will be exemplified below as a prior art that relates to motor drive control. In FIG. 22, each of paper sheets 10 stocked on a paper feed tray 9 is fed into the apparatus by a paper feed portion 11. The leading end of the fed paper sheet is gripped by a gripper 12, and the paper sheet is held on the outer surface of a transfer drum 13. On the other hand, an electrostatic latent image formed in units of colors on a photosensitive drum 14 by an optical unit 15 is developed by a corresponding one of color developers 16 to 19, and is transferred onto the paper sheet 10. The paper sheet 10 is separated from the transfer drum 13, and images formed thereon are fixed by a fixing unit 20. The paper sheet 10, the images on which have been fixed, is discharged onto a discharge tray 37 by a discharge portion 21. The photosensitive drum 14 is cleaned by a cleaning unit 22 to prepare for the next development.
In such apparatus, the paper feed portion 11, transfer drum 13, and the like comprise stepping motors for driving. A drive IC used for driving such stepping motor has ports for setting the current values to be supplied to the individual coils in units of excitation phases, and the excitation current value is determined depending on the voltages to be applied to these excitation phases. Conventionally, motor drive control is done by applying predetermined constant voltages to the respective ports.
However, the above prior art poses the following problems since constant voltages are applied to the current value setting ports of the individual phases.
The excitation current is always constant independently of the load. That is, in order to drive a heavier load, the current must be re-set. On the other hand, when the load is small, unwanted vibrations are produced, and consumption power is wasted due to an excessive torque.
In order to obtain smooth rotation, the individual coils must produce identical torques. However, even when identical voltages are applied to the ports, the individual coils produce different torques due to errors of the coils themselves and the drive IC itself. Hence, torque variations produce unwanted vibrations.
In particular, in a color printer which is demanded to reproduce a halftone image, such torque variations cause banding and color nonuniformity.
Also, in the above prior art, since the stepping motor is controlled by an open loop, the following problem is posed. For example, even when the motor steps out due to, e.g., an excessive load, such errors cannot be detected. Therefore, machine operation hangs up there unless rotation abnormality is detected by an extra sensor or the like. In this case, a current larger than that flows in normal rotation flows, and makes the motor and motor drive generate heat. In the worst case, they may catch fire or produce smoke. Even when abnormality has been detected using an extra sensor or the like, whether the abnormality is caused by a motor failure or sensor failure cannot be detected.